An example of such a hydraulic drive system is the one disclosed in Patent Document 1. The hydraulic drive system of Patent Document 1 has a main relief valve and unloading valve connected to a hydraulic fluid supply circuit through which pressurized hydraulic fluid flows from a hydraulic pump (i.e., main pump). The main relief valve is a type of safety valve and starts to operate when the loads on actuators are high and the pressure in the hydraulic fluid supply circuit (i.e., the discharge pressure of the hydraulic pump) reaches a relief pressure of, for example, 25 MPa during operation of flow control valves, whereby the circuit pressure can be prevented from exceeding the relief pressure. The unloading valve operates mainly when the flow control valves are not in operation (i.e., placed in neutral position) and control the pressure in the hydraulic fluid supply circuit (i.e., the discharge pressure of the hydraulic pump) such that it becomes higher than a target pressure for load sensing control (e.g., higher than 1.5 MPa) and lower than the relief pressure (e.g., set to 2.0 MPa), thereby reducing energy loss when the flow control valves are in neutral position.
Patent Document 2 also discloses a hydraulic drive system that is capable of switching the relief pressure of a main relief valve between a first value (standard value) and a second value for high-load operation which is greater than the first value.